METHODS AND APPARATUS FOR TAMPER DETECTION IN WATERMARKING SYSTEMS

摘要

The invention relates to watermarking systems, which irregularly change the embedded watermark so as to avoid hacking the system by averaging-attacks. In averaging attacks, segments of the watermarked signal are accumulated. This causes the host signal to be cancelled out whereas the embedded watermark accumulates coherently. A watermark A thus determined is then subtracted by a hacker from the watermarked signal. This invention exploits the insight that the hacker does not know when the embedded watermark changes (from A to B, or from A to none). Accordingly, fragments of the hacked signal will contain the negative watermark-A being unintentionally embedded by the hacker. This causes the watermark detector to produce a correlation peak of opposite polarity. The invention resides in the detection of such a negative peak, and concluding therefrom that the signal has been tampered. The payload of the watermark is preserved. This provides the possibility to trace back the hacker.